1. Field of the Invention
The disclosure generally relates to a signal processing technique, and more particularly, to an automatic gain control technique.
2. Description of Related Art
Signals are usually transmitted in a form of radio frequency (RF) signals in wireless communication. For a receiver, it needs to amplify an RF signal and then sample and digitize the analog signal for subsequent processing. If the RF signal is too weak, an optimized digitization resolution may not be achieved. Conversely, if the RF signal is excessively strong, saturation may occur during digitization of the RF signal because the digitization is limited by a valid analog-to-digital conversion range. As a result, the digitized signal may not be recovered to the originally transmitted signal and may also be easily affected by noise interference.
FIG. 1 illustrates a conventional digitization process performed on an RF signal. Referring to FIG. 1(a), two saturation boundaries are indicated with two dashed lines ‘1/Vmax’ and ‘−1/Vmax,’ indicating a valid analog-to-digital conversion range between a positive maximum value Vmax and a negative maximum value −Vmax. Generally, the maximum value Vmax is normalized to 1 such that the valid analog-to-digital conversion range is simplified to be between +1 and −1. Ideally, the amplitude of the amplified analog signal falls between 1/Vmax and −1/Vmax so as to best utilize the conversion range.
Referring to FIG. 1(b), if the RF signal is over-amplified, some segments of it may have amplitudes exceeding 1/Vmax or −1/Vmax and hence become saturated, resulting in digital data being maintained at +1 or −1 with the information of the signal lost. Referring to FIG. 1(c), if the RF signal is under-amplified, the maximum digitization resolution cannot be achieved, even though the amplitude of the amplified signal falls within the non-saturation region.
Conventionally, a target value to be achieved after an amplification process is set to a constant, i.e., the gain is fixed. FIG. 2 illustrates a conventional RF signal amplified with a constant gain. Referring to FIG. 2, the maximum resolution may not be efficiently used in case the RF signal itself is changed by, for instance, environmental affections. For example, in a case where an RF signal is weakened by environmental factors when it is transmitted, the actual digitization resolution under a fixed signal gain may fall between e.g., +0.8 and −0.8, failing to achieve the full maximum digitization range.
How to automatically adjust the signal gain so as to efficiently use the maximum resolution of digitization is one of the essential subjects under consideration.